It's not as unambiguous because, to the guy with his back to starboard, forward could just as easily mean port! It may seem clear if someone's shouting, and pointing, at the bow, but if I'm recounting the action of a tense combat scene and say he "slips on a banana peel and falls 'forward'", you can't know for sure what I mean- does he fall straight towards port, or does he a break a leg, twisting, tumbling toward the bow? c: